The Caribbean region, including the Commonwealth of Puerto Rico, is endemic for diseases caused by the human immunodeficiency virus (HIV) type 1 and also by the human T lymphotropic virus type I (HTLV-I). Due to similarity in their modes of transmission, both viral infections are found predominantly among the intravenous (i.v.) drug abusers. The acquired immunodeficiency syndrome (AIDS) caused by HIV assumes an extremely aggressive form in the Caribbean. The present study proposes to prospectively investigate the possibility that an endemic HTLV-I infection which preceded HIV among the Caribbean residents, might have contributed to produce such an aggressive form of AIDS in this region. Specifically, the study proposes (1) to conduct seroepidemiologic surveys for the retroviral infections among the general population as well as among i.v. drug abusers in order to establish accurate prevalence data; (2) based on the information obtainable through a questionnaire administered in (1), to conduct a case control analysis. Specifically, (a) whether an HTLV-I seropositive subject has a higher risk of also being HIV seropositive; and (b) whether an individual who is co-infected by both retroviruses, either presents a more aggravated clinical disease or has progressed more rapidly than those with HIV infection alone. Subsequently, (3) after selecting subjects from those studied in (1), to conduct a cohort study in order to determine; (a) whether an HTLV-I(+) but HIV(-) subject more readily seroconverts to HIV(+) than an HTLV-I(-); and also (b) whether an HIV(+) subject has a more aggressive disease progression if also infected by HTLV- I, than those without the latter. And finally, to evaluate a co-factor effect of HTLV-I, as compound to those which have also been implicated by other studies, on the AIDS pathogenesis among the Caribbean i.v. drug abusers. Those who are infected by both retroviruses may require a treatment somewhat different from currently treatments for a majority of the US urban cases. The latter are traditionally associated with the homosexual males, and are usually without an HTLV-I complication.